1. Field of the Invention
The present invention relates generally to a pulse width modulated signal amplifier, and more particularly to a pulse width modulated signal amplifier in which oscillation is prevented from being generated when power is supplied.
2. Description of the Prior Art
A prior art pulse width modulated signal amplifier comprises an amplifier in which the amplitude variation of an audio signal is changed to a variable width pulse (which is a rectangular wave with the frequency of about 500 KHz) and then amplified. Since such amplifier is highly efficient, small in size, light in weight and can produce a large output, it is relatively frequently used.
An example of the prior art pulse width modulated signal amplifier is shown in FIG. 1. In this case, its output signal is fed back to reduce distortion.
In FIG. 1, reference numeral 1 designates an input terminal to which supplied is an audio signal. This audio signal supplied to the input terminal 1 is fed through a series circuit of a capacitor C.sub.C and a resistor R.sub.I to the inverted input terminal of an operational amplifier 2a which forms an integrator 2 and whose non-inverted input terminal is grounded. A carrier signal oscillator 3 is provided which produces a carrier signal such as a rectangular signal with the frequency of, for example, 500 KHz. The carrier signal derived from the oscillator 3 is applied through a resistor R.sub.O to the inverted input terminal of the operational amplifier 2a which has its output connected through a capacitor 2b to its inverted input terminal. At the output side of this operational amplifier 2a there is obtained a signal which is provided by integrating a composite signal of the audio signal and carrier signal and which is in turn supplied to the input side of a comparator 4. This comparator 4 is comprised of, for example, a series circuit of inverters 4a, 4b, 4c and 4d and compares the output signal from the integrator 2 with a reference potential such as ground potential. From the output side of the comparator 4, there provided a signal in which the pulse width of the carrier signal varies in response to the variation of the audio signal, that is, a pulse width modulated signal as shown in FIG. 2.
The pulse width modulated signal from the comparator 4 is supplied through a first drive circuit 5p to the gate of a p-channel MOS field effect transistor (which will be hereinafter referred to simply as a MOS FET), which forms a C-MOS inverter 6 or an output amplifier, and also through a second drive circuit 5n to the gate of an n-channel MOS FET 6n, which forms the C-MOS inverter 6 together with the p-channel MOS FET 6p.
The source of the MOS FET 6p is connected to a power source terminal 7p which is supplied with a positive DC voltage +V.sub.CC, while the source of the MOS FET 6n is connected to a power source terminal 7n which is supplied with a negative DC voltage -V.sub.CC. The drains of the MOS FETs 6p and 6n are connected together. The signal obtained at this connection point is fed back through a negative feedback resistor 9 to the inverted input terminal of the operational amplifier 2a forming the integrator 2 to thereby reduce the distortion. An output terminal 8 is led out from the connection point of the drains of the MOS FETs 6p and 6n.
With the prior art example shown in FIG. 1, during the period when the pulse width modulated signal shown in FIG. 2 is negative, the MOS FET 6p turns ON, while during the period where the pulse width modulated signal is positive, the MOS FET 6n turns ON. Thus, at the connection point of the drains of the MOS FETs 6p and 6n, namely output terminal 8, there is obtained the amplified pulse width modulated signal. The pulse width modulated signal thus amplified and derived is demodulated through a low pass filter (not shown) and then if the demodulated signal is fed to, for example, a speaker (not shown), good sound will be reproduced.
According to the prior art pulse width modulated amplifier shown in FIG. 1, before the carrier signal oscillator 3 operates normally when power is applied, the closed loop including the integrator 2 is self-oscillated at the frequency of several MHz to several ten MHz, which frequently results in the generation of pop noises. The reason of this phenomenon is that when the power supply is turned on the output signal from the C-MOS inverter 6 forming the output amplifier is unstable and the output signal therefrom which must be negatively fed back to the integrator 2 is inherently, substantially positively fed back.